Cysteine is used for the purpose of improving food flavor, taste etc. Among the known methods for producing cysteine, which are primarily used at present, are a proteolysis method and a semisynthesis method. In order to use cysteine for improvement of flavor and taste of food, it is required that a natural food material have a high content of cysteine. However, few natural food materials of such kind have been known heretofore. Meanwhile, it has been reported that a food material having a high content of cysteine can be obtained when yeast extract containing γ-glutamylcysteine is heated or treated with an enzyme (WO00/30474).
In Saccharomyces cerevisiae, γ-glutamylcysteine is synthesized by the action of γ-glutamylcysteine synthetase using cysteine and glutamic acid as substrates. Further, glutathione is synthesized by the action of glutathione synthetase using γ-glutamylcysteine and glycine as substrates. Yeasts having a high content of γ-glutamylcysteine have been reported in WO00/30474; Otake et al., Agri. Biol. Chem., 54 (12): 3145-3150 (1990); Chris et al., Molecular Biology of the Cell., 8, 1699-1707 (1997); Inoue et al., Biochimica et Biophysica Acta, 1395, 315-320 (1998) and so forth. However, all of these reports concern investigations using Saccharomyces cerevisiae, and there has been no report about investigations using Candida utilis. 
Candida utilis has been previously classified in the genus Pichia or Hansenula, but is currently classified in the genus Candida. This genus Candida is one of 15 genera of imperfect yeasts, which do not carry out sexual reproduction or have not been found to carry out sexual reproduction, and is only a pickup genus in view of phylogenetic systematics (“Kobo Kenkyu Giho No Shintenkai”, pp. 124-125 (ISBN: 4-7622-4670-0)). Yeasts of the genus Candida often have distinct characteristics in comparison with Saccharomyces cerevisiae. For example, Candida utilis has characteristics of obtaining most of its energy from the pentose phosphate cycle producing a pyridine base (Biotechnology, 3, 30 (1983) (ISBN: 3-527-25765-9)), weak catabolite repression (Biotechnology, 3, 30 (1983)) and so forth. Further, since Saccharomyces cerevisiae is usually used as a research yeast, there have been few findings about Candida utilis. Under such circumstances, there has been no report about how Candida utilis biosynthesizes glutathione, and there have been only reports that a specific Candida utilis obtained by utilizing zinc resistance or the like shows a temperature at which it produces a large amount of glutathione lower than the normally observed temperature by 5° C. or more (Japanese Patent Publication (Kokoku) No. 03-18872) etc.
Thus, the relationship between the glutathione synthetase activity and accumulation of γ-glutamylcysteine in Candida utilis remains unknown, and whether γ-glutamylcysteine can be accumulated by reducing the glutathione synthetase activity has been unclear.